KN4
KN4 is the abbreviation of Kyuubi-Naruto-4 from the Naruto anime/manga. KN4 is the form in which Naruto loses it and starts a rampage. It is so far the greatest number of tails he's able to get when releasing the Kyuubi's Yang chakra.
Naruto's demon fox's cloak form with four tails is called KN4.